1. Field of the Invention
The present invention relates to a traction control system for controlling drive torque of an automotive engine so as to prevent wheel slip of an automobile during acceleration.
2. Description of Prior Art
A traction control system has been known in which the drive torque is controlled to be suppressed in response to detection of wheel slip during acceleration of a motor vehicle. This known system, however, cannot completely eliminate the wheel slip because the control is commenced after detection of the wheel slip.
In order to overcome this problem, a traction control system capable of preventing wheel slip when starting or accelerating a motor vehicle has been proposed in, for example, Japanese Unexamined Patent Publication No. 61-135945. This system detects the state of the road surface and sets a limit drive torque in accordance with the detected state of the road surface, the limit drive torque being the threshold drive torque above which a wheel slip will occur during acceleration. The drive torque acting on the wheel is sensed by a torque sensor and the engine is controlled to maintain the detected drive torque below the limit drive torque, thereby preventing occurrence of the wheel slip during start and acceleration.
This known system also suffers from a problem in that the traction control cannot be performed with high accuracy due to the fact that the drive torque is controlled to a level higher or lower than the command torque or target torque due to inferior accuracy of the torque sensor which detects the drive torque.
In this known system, one of a plurality of previously registered values is set as the limit drive torque in response to, for example, a wiper drive signal or a signal from a switch manipulated by the driver. Actually, however, the limit drive torque should vary according to delicate change in the state of the road surface. Thus, the conventional system cannot optimumly adapt the limit drive torque to a change in the condition of the road surface. In addition, as stated above, the actual drive torque cannot be precisely detected due to inferior accuracy of the torque.
Consequently, the known traction control system often occurs a wheel slip during acceleration due to surplus drive torque or reduces the acceleration due to insufficient drive torque.